This invention relates to over-current controls and, more particularly, to an over-current control which uses integrated circuitry to protect equipment from being damaged during fault or overload conditions.
Electrical systems in residential, commercial and industrial applications often include a panel board for receiving electrical power from a utility source. The power is routed (or distributed) through protection devices to designated branch circuits that go supply one or more loads. These protection devices are typically circuit interrupters such as circuit breakers and fuses that are designed to interrupt the electrical circuit if the limits of the conductors supplying the loads are surpassed.
Fuses provide protection by incorporating within them an element that melts under the application of sufficient current, thus providing a break in the circuit. The break in the circuit acts to stop the flow of current in the circuit, thereby protecting equipment and persons from harm caused by the abnormal situation. The historical advantage of fuses has been their relatively low-cost and simple installation and use. However, fuses are inherently limited by their one-time use characteristic. Once a fuse has xe2x80x9cblown,xe2x80x9d with relatively few exceptions, the fuse must be replaced for the circuit to function in the future.
Circuit breakers, because of their ability to be reset and reused, are a preferred type of circuit interrupter. Circuit breakers are designed to trip open and interrupt an electrical circuit in response to short circuits and overload conditions. Short circuit protection in a circuit breaker is provided by an electromagnetic element that trips when sensing abnormally high current flow in the circuit. The elevated current level causes a high magnetic flux field around a yoke to draw a magnetic armature toward the yoke. The magnetically drawn armature rotates about a pivot and a trip lever, is released from its engagement with the armature. The release and movement of the trip lever causes moveable contacts to separate, thereby interrupting the electrical circuit.
Overload protection in a circuit breaker is provided by a thermal bimetal element that will bend when heated by increased current, causing the circuit breaker to trip and interrupt the power. This can occur when too many loads draw power from the same branch circuit at the same time, or when a single load draws more power than the branch circuit is designed to carry. The bimetal is composed of two dissimilar metals which are laminated or bonded together and which expand at different rates due to temperature increase, thereby causing the bimetal to bend. The yoke and armature described in relation to short circuit tripping, supra, are connected to the bimetal, so that the yoke and armature are carried with the bimetal as it bends. As described above in connection with short circuit tripping, this causes the armature to release its engagement of the trip lever, thereby causing the movable contacts to open and interrupt the electrical circuit.
Thermal current sensing mechanisms have a wide range on their tolerance and are only useful in applications where the coordination of the operation of the circuit breaker is not critical to the rest of the electrical system. These mechanisms also have to be insulated for the full operating voltage of the device since they operate at system voltage. The application of these devices is typically restricted to levels of 1000 volts and below.
Electronic current sensing circuits are applied on both distribution (15-38 kv) and transmission systems (69 kV and above). The electronic circuits are insulated from the high voltage through the current transformer. Electronic current sensing circuits typically have a large number of user configurable settings allowing them to be applied in many diverse application.
There are many applications on distribution systems where it would be desirable to replace older fuse technology with newer circuit breaker technology. In these allocations the large number of settings provided by present electronic controls is not an advantage since the application does not require sophisticated coordination schemes. Also, the cost of the present electronic controls ranges from several hundred dollars to several thousand dollars, depending on the sophistication of the control, which makes the replacement of the older technology cost prohibitive.
It would be desirable for these applications where older fuse technology could be replaced with newer circuit breaker technology to have a cost-effective control which has a limited range of settings to meet the needs of the application. The low-cost control described below provides such a device. The cost of the control described below is in the tens of dollars per phase and provides the necessary settings for fuse replacement applications.
An object of the present invention is to use solid state components in an over-current control to mimic the time response characteristics of a fuse.
Another object of the present invention is to provide self-protection circuitry to prevent damage to the over-current device itself.
Still another object is to reduce the cost of over-current control by incorporating integrated circuitry.
A further objective of the present invention is to enhance the accuracy of the over-current control by providing a means of fine-tuning the over-current trip point.
The present invention is a low-cost over-current control device for providing protection to equipment and persons under fault conditions. The sensing circuit of the invention may be either single-phase or three-phase. The device transforms the primary current of the circuit being protected and the transformed alternating current (AC) is rectified to a proportional DC voltage at a workable level suitable for the remainder of the over-current protection circuitry. A current transformer steps down the operating voltage of the primary circuit, and a load resistor provides a proportional working voltage for the over-current device appropriate for solid-state circuitry.
The preferred embodiment of the invention may be used as a relatively low-cost substitute in locations where fuse characteristics are advantageous but fuse replacement is not. For instance, the present invention could be used in remote locations where the remote operation of equipment is desirable, such as unmanned facilities where it would be extremely costly to dispatch a repair team for a simple fuse replacement. The device will reset automatically and allow for resumption of operations without the need for dispatching a repair team to the remote location. The self-protection circuitry incorporated in the low-cost over-current device ensures that the device is not destroyed during the act of protecting the primary circuit.